I. Field of the Invention
The present invention relates to wireless communication systems and devices. More particularly, the present invention relates to a novel and improved method for rapidly and automatically switching between wireless communication systems in a multiple-mode wireless communication device upon the failure of reverse link communications with a current base station.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in a wireless communication system in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. An example of a TDMA communication system is the pan-European Global System for Mobile Communications (GSM). An example of an analog FDMA system is the Advanced Mobile Phone System (AMPS) presently used in the U.S. for cellular communications.
However, the spread spectrum modulation technique of CDMA has significant advantages over these other techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled xe2x80x9cSOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, Ser. No. 07/433,031, filed Nov. 7, 1989, also assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
In the field of wireless communications, such as cellular, wireless local loop, and Personal Communication Services (PCS), base stations communicate with remote subscriber units such as portable radiotelephones. For simplicity, the term xe2x80x9cmobile stationxe2x80x9d will be used herein to refer to such remote subscriber units, although it is understood that some remote subscriber units, such as wireless local loop phones, do not often move throughout the wireless environment, but rather are generally stationary.
Typically, in any geographic service area, there will be more than one wireless communication service provider. For example, in cellular systems in the United States, there are usually two service providers, one whose system is designated System xe2x80x9cAxe2x80x9d, and another whose system is designated System xe2x80x9cBxe2x80x9d. For the currently planned PCS services in the United States, there are many more service providers, denoted by blocks xe2x80x9cAxe2x80x9d-xe2x80x9cFxe2x80x9d, covering the same geographical service area. The available frequency spectrum for each geographical service area is divided up among these wireless telecommunication service providers. Each service provider typically operates its own base stations and other network equipment.
In accordance with the various wireless communication standards, including Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) Interim Standard IS-95, entitled xe2x80x9cMobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, there exist dual-mode CDMA/AMPS portable radiotelephones which may communicate with either a CDMA base station or an AMPS base station. Furthermore, there are other industry standards existing or in development which provide for dual-mode operation among other modulation and multiplexing schemes such as dual-mode CDMA PCS band and AMPS, dual-mode CDMA PCS band and CDMA cellular band, dual-mode GSM and AMPS, and various other combinations of the known modulation and multiplexing schemes. Other dual-mode mobile stations operate according to one standard when in a first mode, and according to another standard when in a second mode. For example, a dual-mode mobile station may operate according to ANSI J-STD-008, entitled xe2x80x9cPersonal Station-Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systemsxe2x80x9d when in a PCS mode of operation, and according to the AMPS standard when in an AMPS mode of operation.
As can easily be appreciated from the number of co-located service providers and the number of communication protocols that may be used, there are a large number of possible communication systems that may be operating in a single geographic area, each with varying degrees of coverage. For example, since AMPS was the first analog FM-based cellular system to gain wide market acceptance in the United States, AMPS communications systems presently provide nearly 100% coverage for the entire populated regions of the U.S. However, as other competing communication systems such as CDMA cellular and CDMA PCS systems are being deployed, their total coverage area is rapidly expanding. Thus, it is anticipated that there will be many co-located communication systems with varying and overlapping degrees of coverage.
Due to the advantages of CDMA as outlined above, many users of dual-mode CDMA/AMPS mobile stations prefer to use CDMA service whenever it is available and use AMPS service only when CDMA service is not available. Furthermore, a particular user of a dual-mode CDMA cellular and CDMA PCS portable radiotelephone may prefer to use the PCS services over the cellular services for various reasons. For these reasons, dual-mode mobile stations designed in accordance with IS-95 generally allow the user to select a preferred mode of operation (i.e. CDMA or AMPS), and the mobile station will operate accordingly. Other standards may allow the same user-preferences, or may xe2x80x9chard-codexe2x80x9d a particular system priority.
This xe2x80x9chard-codedxe2x80x9d preferences list is often referred to in the art as a xe2x80x9cpreferred roamingxe2x80x9d list. Put simply, a preferred roaming list is a list of systems that are preferred over other system choices when the mobile station is seeking to establish service. The service providers on these preferred roaming lists may have inter-system roaming agreements to facilitate billing and other administrative or operational activities.
In any event, whenever the user of a dual-mode mobile station is in a geographical area where good coverage exists for one of the non-preferred communication systems, for example AMPS, but imperfect coverage for the preferred system, for example CDMA, the user will generally wish to make a xe2x80x9cseamlessxe2x80x9d transition between the two systems without requiring excessive attention to the portable radiotelephone.
In the art, the forward link (base station to mobile station) is generally assumed to fade together with the reverse link (mobile station to base station). However, due to the complexities of network planning, the forward and reverse links may be somewhat imbalanced. That is to say that the forward link may not be receivable by the mobile station in some localized areas where a base station should otherwise be able to receive the reverse link. This first case is generally referred to as being xe2x80x9cforward link limited.xe2x80x9d A forward link limited condition might be caused by an obstruction or reflection in the path of the forward link that might not be as harmful to the reverse link due to the difference in frequency between the forward and reverse links. Conversely, the forward link may be strong enough that it is receivable by the mobile station in some localized areas where the base station is not able to receive the reverse link. This latter case is generally referred to as being xe2x80x9creverse link limited.xe2x80x9d A reverse link limited condition might also be caused by differences in propagation path, or perhaps because the service provider has increased the effective radiated power (ERP) of the base station in order to fight jamming by other nearby base stations belonging to a competing system.
To illustrate the undesirable effects of the reverse link limited condition, consider the case where the dual-mode CDMA/AMPS mobile station is operating in the CDMA mode, and moves into a deep fade area for the CDMA reverse link, such as inside a building or near the edge of a cell""s coverage, and thereby is unable to transmit any messages to the CDMA base station successfully, but would be able to successfully transmit to an AMPS base station if it were operating in the AMPS mode. Further assume that the CDMA forward link is still strong enough in this case to be successfully demodulated by the dual-mode mobile station. In a conventional dual-mode mobile station, a signal strength indication such as a number of signal bars would be displayed on the mobile station""s visual display because of the strength of the forward link. However, if the user attempts to originate a call during this condition, the call will be unsuccessful and a xe2x80x9cCall Failedxe2x80x9d indication will be displayed to the user. Thus, the user will see a satisfactory service indication on the display due to the relative strength of the forward link, but will still be unable to communicate with the CDMA base station.
In this reverse link limited situation, the dual-mode mobile station would be unable to acknowledge a page, originate a call, or even register with the CDMA base station even though a satisfactory service indication is displayed. Furthermore, in a conventional dual-mode mobile station, since the mobile station can successfully demodulate the forward link of the preferred system (CDMA), it would not attempt to acquire the non-preferred system (AMPS), and thus the user would be unable to originate or receive any calls unless the user manually took action to force the phone to acquire the non-preferred system, such as by changing the mobile station to an xe2x80x9cAMPS onlyxe2x80x9d mode and then redialing the outgoing phone number. Also, the user would be unaware of the reverse link limited condition because the conventional dual-mode mobile station would indicate satisfactory service.
Thus, the conventional dual-mode mobile station requires the user at least to take some affirmative action to redial a phone number due to a failed call attempt, and at worst requires the user to access a user-preferences menu to manually switch the mobile station to an alternate system. What is needed is a mobile station that automatically and seamlessly switches to an alternate, available system when it encounters a reverse link limited condition, without requiring the user to take any affirmative actions to select the alternate system or re-initiate a call origination.
The present invention is a novel and improved method for rapidly and automatically switching between wireless communication systems in a multiple-mode wireless communication device upon the failure of reverse link communications with a current base station. The wireless communication device employing the method comprises a processor for generating a first signaling message for transmission to a first communication system having a first pilot signal, and a transceiver for transmitting the first signaling message to the first communication system. In response to the first signaling message, the first communication system should generate a first acknowledgment message. The transceiver receives the first pilot signal and also should receive the first acknowledgment message from the first communication system. However, if the first acknowledgment message is not received, but the first pilot signal is being received (indicating that the wireless communication device is in a reverse link limited situation), the processor automatically generates a second signaling message for transmission to a second communication system having a second pilot signal. Thus, the wireless communication device automatically seeks to acquire the second communication system upon failure to close the reverse link with the first communication system.
In a preferred embodiment, the wireless communication device further comprises a user interface, such as a keypad and associated circuitry, for generating a first set of dialed digits, and a memory for storing the first set of dialed digits, with the first and second signaling messages both including the first set of dialed digits. For example, the first set of dialed digits may be a desired destination telephone number. The destination telephone number is stored in the memory for later retrieval by the processor in order to generate the second signaling message. In such a case, the first and second signaling messages would be call origination messages, and thus the processor automatically xe2x80x9cretriesxe2x80x9d the call origination with the second communication system by retrieving the dialed digits from memory without requiring the user to take any action.
To accomplish this, in the preferred embodiment, the processor comprises a call processing module for generating the first and second signaling messages and for generating an origination hold signal, and a user interface module for storing the first set of dialed digits in the memory and for providing the first set of dialed digits to the call processing module in response to the origination hold signal. The call processing module sets said origination hold signal to the logical value xe2x80x9cTRUExe2x80x9d when said wireless communication device is acquiring the first or second communication system, and to the logical value xe2x80x9cFALSExe2x80x9d when said wireless communication device has acquired said first or second communication system. Thus, when the user interface module senses the toggling of the origination hold signal from TRUE to FALSE, it re-initiates the pending call origination that was held over during the acquisition attempt.
The method of the present invention is a method for establishing wireless communication service with an alternate communication system upon a failure of communication with a current communication system. This method is performed by the mobile station described herein. The mobile station stores a list of preferred communication systems. The mobile station attempts to acquire one of these preferred communication systems upon failure of the reverse link. This may include attempting to acquire the preferred communication systems in a serial order until said one of the preferred communication systems is acquired or until acquisition has been attempted on each of the preferred communication systems at least once. If that is not successful, the mobile station attempts to acquire a communication system most likely to provide service. This may be a neighboring AMPS communication system, for example. If that is also not successful, the mobile station re-attempts acquisition of the current communication system.
In a preferred embodiment, the mobile station also determines whether the alternate communication system is desirable once it is acquired. If it is not desirable, then the mobile station re-attempts acquisition with one of the preferred communication systems after a predetermined idle time.
Thus, the wireless communication device automatically and seamlessly switches to an alternate, available system when it encounters a reverse link limited condition, without requiring the user to take any affirmative actions to select the alternate system or re-initiate a call origination.